A typical post tensioning method tensions and fixes a PC steel material by inserting the PC steel material in a cylindrical sheath buried in concrete beforehand, and applies compressive stress to the concrete by the reaction force of tensioning force. This method covers the disadvantage such that the tensile strength of the concrete is low.
In this post tensioning method, a grout material, such as cement milk, is injected and mixed between the sheath and the PC steel material to prevent adhesion between the PC steel material and the concrete and corrosion of the PC steel material.
Since the operation of injecting the grout material is performed at the construction site, it is troublesome and increases the cost. For this reason, a pregrouted PC steel material, in which the sheath, the PC steel material, and the grout material described above are provided beforehand has been used. This pregrouted PC steel material includes a PC strand formed by twisting a plurality of steel wires (element wires) together, a pregrouted layer disposed on an outer periphery of the PC strand to contain the PC strand, and a sheath that covers an outer periphery of the pregrouted layer (see paragraph 0005 and FIG. 2 of PTL 1).
In the post tensioning method using this pregrouted PC steel material, the pregrouted material (pregrouted layer) is required to have a long tensionable period in which the PC grout material does not cure until the PC strand is tensioned, and needs to cure at ordinary temperature after the PC strand is fixed by the application of tensioning force (after concrete is compressed).
For this reason, to support the action, there have been proposed various kinds of grout materials, that is, a grout material whose composition, viscosity, and so on are determined according to the curing time to cure the grout material in a required time (PTL 1, claim 1), a grout material in which a curing agent is mixed at a mixture ratio in accordance with the curing time (PTL 2, claim 1), and a grout material whose compounding is devised (PTL 3, claim 1).